With modern motor vehicles there is a need to reduce the undesirably produced noises in the motor vehicle as much as possible. Alongside the engine frame, the intake section constitutes a main source of these vehicle noises over a wide range of speeds. The noise in the intake section can be damped by means of several passive Helmholtz resonators integrated in the intake section. Although these resonators designed as plastic molded parts are relatively inexpensive and easy to produce, they take up a large amount of space especially when low-frequency noises have to be damped. As, moreover, today's motor vehicles are exceptionally compact in design, particularly in the engine area, the use especially in the confined engine compartment of such space-consuming resonators causes substantial design problems. A further problem arises from the fact that the engine's sound characteristics are not easily influenced by these passive damping means.
For these reasons there are various methods and devices for influencing noise emissions by electronic means. Such devices for electronic noise suppression are also referred to as Active Noise Canceling (ANC) devices.
In ANC systems the noise being damped is registered by means of a microphone then appropriately filtered and inverted. Inversion is typically effected by means of a 180° phase rotation of the registered noise signal. The noise signal having been thus filtered, amplified, and inverted is fed out by means of a loudspeaker and superimposed on the signal being damped, as a result of which the two noise signals are canceled out owing to interferences.
A device of this type is described in, for example, European patent EP 878 001 B1.
With the aid of a schematic block diagram, FIG. 1 shows the basic principle of an ANC system as employed, for instance, in a motor vehicle. Alongside a microphone 102 for registering a noise signal being damped, an ANC system has an amplifier 110 connected downstream in the circuit for appropriately amplifying this noise signal. Connected at the output side downstream of the amplifier 110 is an electronic regulator 140, an output stage 119 for driving a loudspeaker, and a loudspeaker 105. The regulator 140 contains a digital signal processor (DSP) and a codec circuit. The codec circuit essentially contains an input-side analog/digital converter for digitizing the analog noise signal and a digital/analog converter for converting the appropriately conditioned noise signals back into an analog output signal. The DSP processor performs, among other things, the function of digitally filtering and calculating an output signal.
With the aid of a schematic block diagram, FIG. 2 shows a more detailed representation of an ANC system with a digital signal processor and an output stage of class D design.
The inverted noise signals fed out by the loudspeaker 105 for the purpose of noise suppression are appropriately amplified by the upstream output stage 119. The performance of the loudspeaker 105, and hence the quality of noise suppression, essentially depends on the output stage. The loudspeakers require a maximum loudspeaker power Pmax of approximately 30 watt. Although class AB or B linear amplifiers will, if used as output stages, provide the required performance, their efficiency η is only around 60%. The power dissipation PV generated by the output stage will then be as follows:PV=Pmax*(1−η)=30W*(1−0.6)=12 watt.
In the case of maximum loudspeaker power Pmax of 30 watt and 60% efficiency, the output stage 119 thus generates around 12 watt power dissipation PV, which is mainly manifested through heating of the relevant sections of the circuit. This, however, means that additional, expensive cooling measures are required for the output stage. Furthermore, this extensive heating is a factor which in itself renders such output stages unsuitable for miniaturization.
Class D output stages 119 are therefore typically employed for driving loudspeakers. These offer optimum efficiency and very low power dissipation, so are particularly suitable for miniaturization. A class D output stage 119 of this type has a PWM modulator 136 (PWM=Pulse Width Modulator), a downstream output stage driver 121, and a downstream bridge circuit 120 with power MOSFETs. The analog input signal is first converted by means of the PWM modulator into a digital signal with a fixed frequency and a pulse duty factor proportional to the voltage. This converted digital signal is used to drive the power transistors, with these being operated exclusively in switched mode. This prevents the occurrence of the losses with a principle cause that ensue in the case of linear output stages when the supply voltage is divided between the transistor and loudspeaker.
Class D output stages are today typically produced as fully integrated circuits. However, a problem associated with the use of class D output stages is that they are relatively expensive to produce owing to the degree of accuracy required for the PWM modulator. This is compounded by the fact that signal processing in the codec circuit 109 requires a digital signal processor 108 specially provided for this which, moreover, is hardly ever employed for any other functions. However, the use of a digital signal processor 108 solely for the purpose of noise suppression makes the entire system more expensive.
Finally, in order to meet the accuracy requirements for driving the PWM modulator it is necessary to employ an automobile-compatible 12-bit digital/analog converter at the output of the codec circuit 109. To allow fast and hence dynamic conversion, a parallel converter is typically employed here which, however, is relatively expensive.
The functions of a DSP and of a AD converter could, for example, also be performed by a microcontroller as this already has an analog/digital converter on the input side and, in the case of, for instance, a 32-bit microcontroller, offers excellent real-time performance. Furthermore, the functionality of a digital signal processor could be advantageously combined here with other functional properties, for example an engine control or a control device, which would have considerable cost advantages and permit the entire electronic circuit to be significantly miniaturized.
Microprocessors are not, however, used at present for driving a loudspeaker for noise suppression proposes. The reason for this is the necessary resolution of the drive signal of the loudspeaker of 12 bits and a necessary clock frequency of the microcontrollers—currently available for automotive applications—of approximately 40 MHz. The PWM clock frequency fPWM derived from this is as follows:fPWM=40 MHz/212=9.76 kHz.
Although class D output stages can be operated with a PWM clock frequency of approximately 10 kHz, the filter elements at the output of the output stage would then be exceptionally large. Moreover, a higher-order output filter would then be necessary because the PWM frequency is in the audible range, which would in turn necessitate a lowering to below the threshold of perception.
However, PWM frequencies of approximately 200 kHz are required for ANC systems for noise suppression purposes because a highly advantageous compromise can be achieved here between the costs relating to the size of the filter elements on the one hand and the losses due to switching reasons of the power transistors of the class D output stage on the other hand.